The light-evoked K ion conductance increase of the plasma membrane of Aplysia giant neurons is mediated by the release of Ca 2 ion from cytoplasmic pigmented granules. The granules also contain a pigment which absorbs at 450-490nm; this corresponds to the electrophysiological action spectrum. Hence, the neuron is a reasonable model for the much smaller vertebrate rod in which the photopigment is also remote from the plasma membrane. We plan to count the number of granules per neuron, the average elemental content, particularly Ca 2 ion, of illuminated and non-illuminated granules, and the total Ca 2 ion of the neuron. An estimate of the Ca 2 ion stores in the granules and the effects of light upon them will be made using the electron microprobe. The kinetics of the light response will be related to the Ca 2 ion release and followed with the Ca 2 ion photoprotein aequorin. Levels of free Ca 2 ion can be assessed from intracellular pressure injections of Ca EGTA buffers and Ca 2 ion microelectrodes. Absorbance and absorbance difference spectra of individual granules will be analyzed using a microspectrophotometer. Further studies on the particular K ion conductance mechanism in the giant neurons will be done since light is such a convenient stimulus. The nature of the depolarizing light response of certain other Aplysia neurons will also be examined, particularly changes in plasma membrane ionic conductances. The role of Ca 2 ion and the pigmented granules will also be investigated in these neurons.